What are the major classes of second messengers?

The four major classes of second messengers include cyclic nucleotides, lipid messengers, ions that signal within cellular compartments, and gasses and free radicals that can signal throughout the cell and to adjacent cells. Examples of cyclic nucleotides include cAMP and cGMP, which serve as second messengers in both ion-channel and hormone signaling in eukaryotes. Lipid second messengers contain two acyl chains. Some examples are DAG, C1P, and ceramide which always remain associated with the membrane. Lipid second messengers with one acyl chain can dissociate with membranes. Ions such as Ca2+ play an important role in stimulus-response reactions of cells as a second messenger. They achieve this by keeping cytoplasmic Ca2+ concentration low at rest and readily transporting Ca2+ in response to stimuli which activates the cellular reaction. All cells use calcium ions to regulate their activity in response to intrinsic and extrinsic stimuli. Gasses and free radicals such as nitric oxide are another class of second messengers. Nitric oxide permeates the plasma membrane, and thus can travel through the extracellular medium and act within other adjacent cells. NO also activates cGMP by interacting with the heme group of the enzyme soluble guanylate cyclase.